1. Field of the Invention
This invention relates to automatic flight control systems, and more particularly to an electronic automatic approach to hover system in which flight parameters are calculated throughout an approach to hover at a desired destination and altitude.
2. Description of the Prior Art
The utilization of a helicopter frequently involves a hover maneuver in which the helicopter is maintained at a fixed point with respect to the earth, and at a low altitude. In some operations, such as rescue operations, servicing sonobuoys, and the like, the maneuver may need to be performed over water, and frequently at night. This renders maneuvering of the helicopter into a desired hover position very difficult without the aid of automatic navigational aids operable in response to inertial and other sensors. In helicopter flight control systems known to the art, only limited assistance is provided to the pilot. Specifically, TACNAV computers provide visual indications to the pilot with respect to courses and distances to desired waypoints or destinations, but these do not automatically control the flight of the aircraft. In addition, the pilot can receive assistance from an autopilot system which can provide controlled rates of deceleration and descent and roll angles in turns, and the like. In order to provide a stable hover, without having the aircraft pitched upwardly, or rolled to one side, it is known to be desirable to establish hover with the aircraft heading against the wind. In the past, to achieve such a hover, the pilot would have to fly the aircraft some distance downwind from the desired destination point, while observing the indications from his TACNAV computer, until he found himself to be at a desired distance downstream from the destination point such that he could decelerate and descend from his present speed and altitude safely to a hover at a desired altitude over the destination point. In prior art systems, the automatic pilot has provisions for an automatic rate of descent and an automatic rate of deceleration which can be engaged; but utilization of these sytems so as to end up at zero speed at a desired altitude over the destination point requires the pilot to estimate the point in time when he will be entering a window which will cause him to reach the desired point of hover at zero speed with the fixed rates of deceleration and descent provided by the automatic pilot system. No computation of the actual distance, or comparison with actual speed, and no corrections for disturbances in the actual flight profile which is achieved, have been available in the art. If the aircraft in fact descends more quickly than the fixed rate of descent established by the automatic pilot system (which could occur due to a down draft, or an increase in wind speed), then the pilot could achieve the desired hover altitude while still traveling at an excessive speed, causing dangerous high speed flight too close to the earth's surface. On the other hand, if descent is too slow with respect to deceleration, the pilot could be flying over the desired destination point at too great an altitude, in which case the aircraft would substantially sink to the desired altitude with high vertical velocity. Since these maneuvers are being performed under extremely dangerous conditions, where the velocity and altitude relationships are adverse with respect to any engine failure recovery procedures which may be necessary, psychologically adverse maneuvers such as rapid rates of descent or low altitude, high speed flying are to be avoided.